The backpanel on the inside is unable to hold a screw. The metal has worn away.
Tom from SWISCO responded:
May 2, 2018
Greetings from everyone here at SWISCO! It looks like you're in need of a new back plate. Take a look at our 82-108 and compare it to yours. If you would like a complete handle, check out our 82-080 set. Hope this helps!